1. Field of the Invention
The present invention relates generally to power supplies for computer systems. More specifically, the invention relates to switching power supplies for computer systems. More specifically still, the present invention relates to generating phase control signals for switching power supplies having multiple phases.
2. Background of the Invention
As computer system technology advances, specifically as manufacturing techniques related to microprocessors or central processing units (CPUs) advances, more and more transistors and related functionality are placed on a single die of a CPU. As more transistors are placed closer together on semiconductor substrates, less insulation material (in the form of oxide layers) exists between each transistor. Accordingly, CPU operating voltages are lowered to protect against electrical breakdown between transistors. However, adding transistors to a CPU increases the amount of electrical current the CPU requires. Thus, while the operating voltage for CPUs is generally dropping as technology advances, required operating currents are steadily rising. For example, Intel Corporation's most recent processor named the Pentium.RTM. 4 may have operating voltages in the range of 0.9-1.7 volts. Operating voltage for a particular processor, even in the same series, may differ depending on the characteristics of the particular CPU. While Intel's Pentium.RTM. 4 processor has a low voltage requirement, the processor may require as many as 50-60 Amps of current at peak loads. Compared to Intel's Pentium.RTM. II processor, which required only 20 Amps or less, this new processor as much as triples the amperage demand. Every computer system has a power supply that converts the 120 Volt alternating current (AC) found in a standard wall receptacle to suitable direct current (DC) voltages. This conversion from AC to DC is typically done by a switching power supply. A switching power supply supplying power to the Pentium.RTM. 4 processor should be capable of supplying current swings having transient response in the range of 100 Amps per micro-second. Prior CPU's may have required as little as 50 amps per micro-second transient response. Thus, there are increasing demands on the capabilities of switching power supplies with each advance in CPU technology. Other microprocessors may have similar power requirements, for example an AMD Athlon.RTM..
FIG. 1 shows an exemplary partial electrical schematic of a single phase blick-type switching power supply. The circuit shown in FIG. 1 is said to have only a single phase because it has only one switch and inductor combination. If there were several of these switch and inductor combinations present, the power supply would be considered a multi-phase switching power supply. Buck-type switching power supplies are designed to provide lower direct current (DC) voltages than applied at their inputs while supplying the current demand of a load (c.g., CPU). Power supplies provide reduced DC voltages by "chopping" the supply voltage (i.e. turning on and off at a particular frequency )via switch 1 and then averaging, by means of an inductor/capacitor circuit 2, the chopped voltage to produce DC voltage at the desired level.
In the early days of microprocessor technology, a computer system switching power supply may have had only a single phase, as explained above, inasmuch as the processor voltage and amperage requirements were such that a single phase switching power supply was capable of producing the desired voltage with the desired current. While a single phase switching power supply may be capable of meeting average voltage and current requirements, a single phase alone may not be capable of meeting higher transient requirements of modern CPUs. Another consideration in switching power supply design, especially as related to power supplies mounted on a motherboard, is the amount of space required to implement such a supply. If a single switching phase is used, the inductor and capacitor in the averaging portion of the circuit may need to be excessively large occupying too much space on the motherboard.
Increasing the number of phases in a switching power supply permits the capacitor and inductor in each phase to be smaller, as is well understood in the art. Thus, in response to demands such as these, manufacturers generally have increased the number of phases in switching power supplies to meet increasing amperage requirements, space limitations and transient response requirements. However, there are various problems associated with having multiple phases in a switching power supply including how to generate phase control signals as the number of phases increases.